Direct conversion, or homodyne, radio front-end architectures have become very popular, particularly for mobile terminals, because they are power efficient and cost effective. A direct conversion receiver downconverts a received radio frequency signal directly to baseband without any use of intermediate frequencies. As such, the number of circuit components needed for the direct conversion receiver is reduced as compared to a conventional heterodyne receiver. Similarly, a direct conversion transmitter upconverts a baseband signal directly to a radio frequency signal without any use of intermediate frequencies. As such, the number of circuit components needed for the direct conversion transmitter is also reduced as compared to a conventional heterodyne transmitter.
However, direct conversion front-end architectures often suffer from a DC-offset of an unknown magnitude, where the DC offset is a by-product of the direct conversion process. The DC offset mainly stems from three sources: (1) Local Oscillator (LO) signal leaking to, and reflecting from, the antenna and self-downconverting to DC through a mixer used for downconversion, (2) a large near-channel interferer leaking into the LO and self-downconverting to DC, and (3) transistor mismatch in the signal path. The leakage in (2) and (3) can be reduced to some extent by careful front-end design. Nevertheless, if the DC-offset is not completely eliminated in the receiver front-end, then baseband processing must compensate for the remaining DC-offset. This often causes undesirable complication in various aspects of the baseband algorithm design, especially for an advanced receiver, and leads to an increase in complexity and cost of the receiver.
To avoid dealing with the DC-offset issue, in many Orthogonal Frequency Division Multiplexing (OFDM) based wireless communication standards where the frequency band is subdivided into multiple subcarriers, the subcarrier at the DC tone (i.e., the DC subcarrier) is left idle and is not used for communications between base stations and mobile terminals across the entire network. For example, the DC subcarrier is left idle and is not used in Long Term Evolution (LTE) and WiMAX wireless communication standards. This is clearly not an efficient solution for utilizing the DC subcarrier, which is a valuable radio resource.
As the unknown DC offset typically stays substantially constant for a number of transmission time periods, U.S. Pat. No. 7,773,679 to Laroia et al. discloses a base station in an OFDM-based communication system that every so often does not transmit on the downlink DC tone, or DC subcarrier, while continuing to transmit on other downlink tones. A mobile terminal measures received signal on the downlink DC tone during the time in which transmission is suspended on the downlink DC tone (i.e., the suspended DC tone transmission period) to thereby estimate the DC offset resulting from self-interference at the mobile terminal. The mobile terminal then compensates for the DC offset by subtracting the estimate of the DC offset from the signal received during the time in which transmission is active on the downlink DC tone (i.e., the active DC tone transmission period). A drawback of this method is that there is a residual DC offset after compensation that relies heavily on the accuracy of the estimate of the DC offset. This residual DC offset results in performance degradation.
Further, in order for the mobile terminal to obtain adequate accuracy for the estimate of the DC offset, the suspended DC tone transmission period is required to be rather long, which in turn limits the achievable spectral efficiency. Moreover, all the signals received over all symbol periods also need to be buffered until an accurate estimate of the DC offset can be generated and compensation of the DC offset can begin.
U.S. Pat. No. 7,773,679 to Laroia et al. also discloses that, for the uplink where the DC offset in the signal received by the base station may be attributed to multiple scheduled mobile terminals (as well as the base station if a homodyne receiver architecture is used), a special symbol that is equal to the negative average of the previously transmitted N−1 data symbols is transmitted in every N-th symbol, where N is a predetermined integer. The receiver at the base station estimates the unknown DC offset from a weighted sum of the signals received over a frame of N symbols. This method again requires the implementation of a separate module for DC estimation, and the receiver performance is directly affected by the accuracy of the DC estimate.
As such, there is a need for systems and methods for transmitting data in the presence of interference (e.g., a DC offset) that has an unknown magnitude that is substantially constant without requiring estimation of or compensation for the interference at the receiver.